Rotary drum debarkers are widely employed to remove the bark from logs. The rotary drum debarker is essentially a long cylindrical shell into which the logs are introduced. The drum rotates which causes the logs to tumble and to rub against one another. The tumbling and rubbing action serves to separate the bark from the log so as to prepare the log for further processing, for example as pulpwood in the manufacture of paper.
The shell of the debarking drum is typically thin with slots for the bark to fall through. The interior of the shell is lined with generally longitudinally oriented lifters. The lifters may take the form of L or V shaped angle irons or channels which are welded to the interior of the shell. It should be understood that the lifters may take many shapes and be oriented in various ways in the interior of the drum. The lifters are typically oriented so that the peak or vertex of the lifter is pointed generally toward the axis of rotation of the drum and the legs of the lifter are welded to the inside surface of the shell of the drum. The lifters serve to increase the strength and rigidity of the drum and also serve to lift the logs in the drum as the drum rotates thereby enhancing the tumbling action and therefore the efficiency of bark removal from the logs introduced into the rotary drum debarker. Reinforcing rings may be added to the shell at the inlet and outlet. The reinforcing rings may be welded to the ends of the lifters to reinforce both the lifters and the shell itself.
The tumbling action of the logs produces considerable stress on the structure of the drum and particularly the lifters. Although the lifters are substantial and designed for the severe environment in which they operate, it is inevitable that the lifters will wear and sustain other damage from the tumbling logs. The vertex of the lifters are most exposed to the action of the tumbling logs and therefore suffer the earliest and worst wear. In time the vertex of a lifter may even wear completely through. When this event occurs, or even before this event, it is necessary to replace the lifter to avoid substantial weakening of the structural integrity of the relatively thin walled shell of the drum and to maintain the efficiency of the lifting action.
Repair of a damaged lifter normally requires replacement of the lifter, which in turn entails cutting all or part of the damaged lifter from the drum and welding a new lifter in place of the damaged lifter. This process is difficult, time consuming, and most importantly, requires that the entire rotary drum debarker be taken out of service while the replacement is being effected. In some facilities, taking a debarker is out of service means that the entire facility is either out of service or its operation is substantially curtailed. There is therefore a strong incentive to avoid the necessity of replacing worn or damaged lifters.
Various configurations of lifters are known in the art. For example, U.S. Pat. No. 3,286,747 to Delcellier discloses a debarking drum with various inner surface corrugations including sawtoothed (either sharp edged or rounded) and dimpled variations.
U.S. Pat. No. 5,839,490 to Kiedaisch et al. discloses a debarking drum with a rubber lining for a corrugated inner surface. Kiedaisch et al. note that metal lifters on debarking drums are often lined with rubber to extend the life of the lifters. Kiedaisch et al. also disclose reinforcement members 50, 150 which extend from the leading edge to the trailing edge of each rubber liner and which are stated to extend the life of the liner.
U.S. Pat. No. 1,289,447 to Guettler discloses a drum debarker whose walls are formed of a number of angle bars. The angles project into the interior of the drum and form lifters.
U.S. Pat. No. 3,746,063 to Smiltneek discloses a method for debarking logs wherein a number of high density objects, such as steel slugs, are allowed to randomly impact the logs to assist in loosening the bark. Smiltneek otherwise discloses a fairly conventional debarker drum where the lifters are longitudinal vanes of a triangular shape and with a sharp edged vertex.
U.S. Pat. No. 5,197,524 to Clarke-Pounder et al. relates to a method of introducing steam through longitudinal lifters (staves 50). The staves are shown in cross section as a triangular shape with a rounded vertex.
U.S. Pat. No. 5,019,123 to Clarke-Pounder et al. primarily relates to a discharge control system. Longitudinal lifters (staves 17) with a conventional triangular cross section are disclosed.
U.S. Pat. No. 3,955,608 to Smiltneek discloses a debarker drum with longitudinal vanes 38 which are square or rectangular in cross section.
U.S. Pat. No. 3,807,469 to Schynder discloses a debarking drum with pusher elements 52 which are triangular in cross section. The triangular cross sections are not symmetrical in that the vertex is displaced away from the direction of rotation of the drum.
U.S. Pat. No. 1,311,226 to Guettler disclose lifters which are rounded corrugations. The purpose of the rounded shape is to avoid brooming caused by sharp edged lifters.
U.S. Pat. No. 4,784,197 to Alander et al. discloses a method of heating a debarking drum. The drum is shown with distribution pipes 12 which appear to serve as lifters as well as means for introducing steam. The cross sectional shape appears to be triangular with a rounded vertex.
U.S. Pat. No. 4,369,823 to Gustafsson discloses a drum with a series of sections each having varying numbers of log lifters. The lifters appear to be half cylindrical in cross section.
U.S. Pat. No. 3,896,863 to Smiltneek discloses a debarking apparatus with a number of offset debarking chambers. Longitudinal triangular lifters are shown.
U.S. Pat. No. 3,417,796 to Leider et al. discloses a debarker that may include longitudinal lifters which appear to be somewhat triangular or rectangular in cross section with a rounded vertex.
U.S. Pat. No. 3,269,438 to Guettler discuses the problem of weld failure in "barking irons." "Barking irons" are described as longitudinal members with cross sections that may be M-shaped, U-shaped, etc. The solution offered by Guettler is to replace the "barking irons" with corrugated plates having fewer exposed welds.
U.S. Pat. No. 2,665,721 to Busch discloses a debarking drum with fluted metal slats which may serve as lifters.
The limitations of the prior art are overcome by the present invention as described below.